Preclearance happens for an entire departing flight at a time.
The flight from Abu Dhabi to Dublin will have many passengers on it who are not going to the US, so it would make no sense to preclear that.
(Or in other words, preclearance can only happen for flights whose next stop is in the USA).
Whether you will be precleared in Dublin depends on whether your flight is with an airline that uses the preclearance facility there. Either they do or they don't; as a passenger you don't get to choose to preclear or not. And if you do get preclearance in Dublin, you will find it simply by following the signs towards your announced departure gate. It's not something you can miss.